In normal analogue telephone switching offices, the routing of inter-office telephone calls takes place over fixed paths referred to as trunk lines (or trunks). Only a finite number of direct trunk lines exist between any two telephone switching offices and it is not uncommon to find that no idle direct trunk lines are available between the two switching offices. At this point, the next step is normally to select an idle outgoing trunk to a tandem switching office. The call arrives at this tandeming or intermediate office on an incoming trunk and is then redirected, or tandemed, to an idle outgoing trunk destined for the terminating office. The originating office is operating under the basic assumption that an idle outgoing trunk exists between the tandem office and the terminating office. A blockage in the tandeming office, because of no idle outgoing trunks to the terminating office, wastes processing time in both the originating and the tandeming offices.
Normally, the trunk routing from the originating office to the terminating office is fixed by the system design. That is to say, the originating office follows a fixed routing hierarchy to select an idle trunk. The originating office first attempts to find an idle direct trunk to the terminating office; if there are no idle direct trunks, the originating office attempts to find an idle outgoing trunk to a tandem office according to the system hierarchy. If there are no idle outgoing trunks to a tandem office the calling subscriber who originated the call receives an overflow (e.g. fast busy) signal.
The present invention alleviates some of the problems of this regimented hierarchial type of routing by grouping a plurality of switching offices (e.g. class 5 stored program control offices) into clusters and by providing each switching office with direct trunks to all (or most of) the other switching offices in its cluster. This then allows each switching office in the cluster to serve the dual function both of an originating (or terminating) office and of a tandeming office for its own cluster. Suitable equipment monitors the busy status of all the switching offices in the cluster, and determines a most likely alternate routing scheme for each switching office. The alternate routing scheme for each particular switching office is stored at that particular office and is periodically updated, by suitable equipment, so as to account for changes in the busy status of the other switching offices and trunk lines in the cluster.
Stated in other terms, the present invention is an alternate routing system for a plurality of telephone switching offices, the system comprising: at least one group of trunk lines connecting any one switching office in the system to any other switching office in the system; a first plurality of storage locations, at each switching office, for storing an indication of the terminating office to which a call is directed; a second plurality of storage locations, at each switching office, for storing an indication of the switching office most likely to be available as a tandeming office between the originating switching office and the terminating switching office; a third plurality of storage locations, at each switching office, for storing an indication of the trunk lines to be used for accessing the switching office indicated by the second plurality of storage locations as being the tandeming office; and apparatus for periodically updating the information stored in the second plurality of storage locations, concerning the switching office to be employed as a tandeming office.
Stated in yet other terms, the present invention is an alternate routing system for a plurality of telephone switching offices, the system comprising: at least one group of trunk lines connecting any one switching office in the system to any other switching office in the system; each switching office including a first plurality of storage locations, a second plurality of storage locations, and a third plurality of storage locations; the first plurality of storage locations, at each switching office, storing, as an addressed location for the second plurality of storage locations, an indication of the terminating office to which a call is directed; the second plurality of storage locations, at each switching office, storing, as an addressed location for the third plurality of storage locations, an indication of the switching office most likely to be available as a tandeming office between the originating switching office and the terminating switching office; the third plurality of storage locations, at each switching office, storing, as an addressed location for the first plurality of storage locations, an indication of the trunk lines to be used for accessing the switching office indicated as being the tandeming office by the second plurality of storage locations; and apparatus for periodically updating the information stored in the second plurality of storage locations, concerning the switching office to be employed as a tandeming office.
Stated in yet again different terms, the present invention is an alternate routing method for a plurality of telephone switching offices interconnected in a system configuration wherein any two switching offices in the system are interconnected by at least one group of trunk lines, the method, at an originating switching office, comprising: storing, in a first addressable storage location, an address indicative of a terminating switching office; storing, in a second addressable storage location addressed by the contents of the first storage location, an address indicative of a tandeming switching office to be employed intermediate said originating and terminating switching offices; storing, in a third addressable storage location addressed by the contents of the second storage location, an address indicative of trunk lines to be employed in accessing the tandeming switching office; and periodically updating the information stored in the second addressable storage location.